1. Field of the Invention
The present invention relates to an image reading device which is provided in an image forming apparatus such as a multi-function printer (MFP) or a copier and which performs shading correction on image data obtained by document reading. The present invention also relates to an image reading device shading correction method.
2. Description of Related Art
An image reading device such as a scanner reads a document to generate image data. Moreover, the image reading device may be provided in an image forming apparatus such as a copier, a multi-function printer (MFP), a fax machine, or the like. The image reading device has distortion occurring depending on a pixel position due to a variation between light receiving elements in an image sensor (for example, CCD), a difference in the degree of condensation between a lens center and a peripheral part, uneven light amount distribution of a light source, etc. Thus, in the image reading device, image processing of shading correction for performing, for example, normalization between a white reference and a black reference to correct the distortion depending on the pixel position may be performed on each of pixels.
In order to acquire the white reference (“sparkling white”) in the shading correction, a white reference plate may be provided in the image reading device. Then a white reference of each pixel is defined based on output data provided from the image sensor when the white reference plate has been read. Note that in order to indicate “jet black”, a black reference plate may be provided in the image reading device.
Here, for example, when a character-dominant document has been read, in order to sharpen characters to obtain an image with a clear white-black difference, image processing such as Υ conversion processing, spatial filter processing (for example, edge enhancement processing), and/or the like may be performed on image data already subjected to the shading correction.
Known as one example of an image processor performing such image processing is an image processor which handles image density as image data quantized in multi-gradations for each pixel, which has a Υ converter for the image data and a smoother for the image data, and which controls Υ conversion properties of the Υ converter in accordance with a result of smoothing by the smoother. With this configuration, an attempt is made to obtain a reproduced image with less background disturbance and a clear black-white difference.
Typically, gradations of each pixel of the image data in reading by the image reading device are regulated between a white reference and a black reference. Then the white reference is acquired by reading a white reference plate. In other words, the image reading device cannot read (acquire) white whiter than the white reference plate.
Then in the image reading device, not only a written document but also various other types of documents such as photos are read, and thus it is preferable that a density range in reading be as wide as possible. Therefore, to the image reading device, a whitest possible white reference plate is fitted. Moreover, used as the black reference is, for example, output data of an image sensor in a state in which a reading lamp is turned off.
However, in a case where a character-dominant document is read by a copier or the like, if black as the black reference is darker (blacker) than black of a character portion, characters are read in gray a little brighter than jet black. Similarly, if the white reference plate is brighter (whiter) than background and a blank space of white paper, the background, etc. are read in gray a little darker than sparkling white.
Here, in the image forming apparatus or the image reading device, in a case where a character mode that prioritizes character reproducibility has been selected, in order to obtain an image with a clear black-white difference, image processing of replacing a density value of the pixels at the character portion to be blacker and replacing a density value of the pixels at the background portion to be whiter may be performed. Specifically, the γ conversion processing and/or spatial filter processing as described above may be performed on the image data. Moreover, for example, image processing of replacing a density value to be sparkling white or jet black with density between the white reference and the black reference serving as a border may be performed on each pixel.
However, the image processing such as the Υ conversion processing and/or the spatial processing is performed on the entire image data. Thus, the image processing such as the Υ conversion processing and/or the spatial processing causes a problem of image quality deterioration, such as lack of information of a pixel portion in gradations between the white reference and the black reference, such as pictures, figures, color gradation, a photo portion, etc. other than characters in the image data. In other words, as long as some image processing is performed, the image data is modified, whereby the image quality deterioration possibly occurs in terms of document reproducibility, etc.
For conventional image reading devices, MTF correction (edge enhancement), γ conversion, and/or the like is performed on image data already subjected to shading correction. Therefore, there possibly arises a problem of image quality deterioration attributable to the image processing such as the γ conversion, which fails to solve the problem described above.